The continuous extrusion and production of blown film bubbles in the manufacture of films is well known. Thin films are formed by extruding the desired film composition as a hot melt through a die, while stretching the extruded melt by directing cooling air currents thereon. The melt composition is extruded in the form of a tube, which is drawn by the cooling air flow and nipped at a desired length to form a cylindrical bubble. As the film bubble forms, the polymer cools as it emerges from the die until the polymer achieves sufficient melt strength to stabilize the bubble and prevent its further expansion. The point from the die at which the blown film bubble cools sufficiently to pass from an unstabilized state, where the bubble may be expanded, to where the blown film bubble stabilizes is referred to as the frost line. Once the film bubble has cooled, the bubble is then collapsed at a desired point by nip rollers, and the film is wound onto a spool or the like. The film is thus formed by continuously producing the blown film bubble by simultaneously extruding the melt from the die, directing air currents over the softened and unstable portion of the film at a sufficient air velocity to draw and stretch the film into the bubble having a uniform and constant thickness, while nipping the cooled stabilize portion of the film bubble.
Typically blown films are formed from polymer resins which include one or more of low and high density polyethylene or other thermoplastics. Depending upon the composition and the production rate of the film, the stabilized portion of the blown film bubble may have an elongated length of between 2 to more than 20 meters. Similarly, the diameter of the blown film bubble may vary from only a few centimeters to several meters or more, depending on the intended film applications.
To increase the rate at which the bubble reaches a point of stabilization at the frost line, and thereby increase film production rates, U.S. Pat. No. 4,846,645 to Cole discloses one known bubble forming apparatus. In the apparatus of Cole, the bubble is formed by directing cooling air streams simultaneously on both interior and exterior unstabilized surfaces of the film bubble as the film emerges from the die. It has been found, however, that when the film is produced at very rapid production rates, the film may have cooled insufficiently as it moves through the nip rollers. If this occurs, on pressing the film bubble flat, the nip rollers may cause the film to stick to itself or "block".
The problem of blocking is particularly acute where larger blown thin film bubbles are produced and/or where the blown film bubble is extruded upwardly as a vertically elongated bubble. In particular, it has been found that a pocket of stagnant hot air may form towards the upper area of the bubble, adjacent the area of the apparatus where the film is collapsed by nip rollers. As production rates for the film increase, there exists a corresponding increase in the temperature of the hot air which, if hot enough, may re-heat the film, causing it to stick to itself or block, when the bubble is collapsed and squeezed by the nip rollers.